As an example of an LC oscillator with a resonator consisting of an inductor and a capacitor, such oscillator described in Japanese Patent Application Laid-Open Publication No. 2004-260301 is known. A description about phase noise power of an oscillator in a small signal model is found in D. B. Lesson, “A Simple Model of Feedback Oscillator Noise Spectrum,” Proc. IEEE, vol. 54, pp. no. 2, 329-330, February 1966. Furthermore, a description about phase noise power in a large signal model is found in A. Hajimiri and T. H. Lee, “A general theory of phase noise in electrical oscillators,” IEEE J. solid-State Circuits, Vol. 33, pp. 179-194. Meanwhile, an example of an LC cross coupled oscillator from which a tail current source was removed, in which a common ground point of a differential amplifier is directly coupled to a circuit ground, is described in P. C. Huang, “A 131 GHz Push-Push VCO in 90-nm CMOS Technology”, IEEE RFIC, 2005. An example of an LC cross coupled oscillator without a tail current source is also described in T. Song, “A 5 GHz Transformer-Coupled CMOS VCO Using Bias-Level Shifting Technique”, IEEE RFIC 2004.
Moreover, an example of an oscillator circuit wherein improving a phase noise characteristic has been done by approximating a series resonance frequency of a parallel resonator circuit to a parallel resonance frequency ultimately is described in Japanese Patent Application Laid-Open Publication No. 2004-023762.